The ultimate aim of the proposed study is to develop a reliable bedside test to detect pulmonary aspiration of gastric contents. Such a test is of especial importance in critically ill, mechanically ventilated patients receiving tube feedings. While obvious large-volume aspirations occasionally occur in these individuals, micro-aspirations of regurgitated gastric contents are far more common. These micro-aspirations are usually clinically silent until they cumulatively lead to pulmonary problems (such as chemical pneumonitis or potentially life-threatening aspiration pneumonia). At present, there is no reliable bedside test for aspiration. Unfortunately, without such a test, nurses are unable to distinguish between frequent and infrequent silent aspirators. This, in turn, impedes their ability to intervene in a timely fashion to prevent poor patient outcomes. Our research team has developed a highly sensitive and specific laboratory assay for the major gastric enzyme, pepsin, which serves as a proxy for the aspiration of gastric contents into the lung. The test has been used effectively in a research setting to test for aspiration in over 500 mechanically ventilated patients. Although our laboratory assay provides accurate and useful results, it takes about 24 hours to perform. The proposed study will attempt to refine our laboratory assay for use at the bedside within a 5 minute period. The assay will be based on new technology (referred to as molecular pincer technology), developed by the biochemistry collaborators on the proposed study. The new antibody-based sensor design allows rapid and sensitive detection of target proteins with significantly fewer steps than required for laboratory assays of the same proteins. A total of 2,200 aliquots of human tracheobronchial secretions are available for the project and will be used to determine measures of sensitivity, specificity, and reproducibility when comparisons are made between the bedside assay and the reference laboratory assay. Public Health Relevance: The study's aim is to develop a bedside assay to detect pulmonary micro-aspirations of gastric contents. Such a test is needed to differentiate between frequent and infrequent aspirators so that interventions can be planned accordingly. A total of 2,200 aliquots of human tracheobronchial secretions are available for the project and will be used to determine measures of sensitivity, specificity, and reproducibility when comparisons are made between the bedside assay and the reference laboratory assay.